This invention relates to patch filter elements suitable for use in stripline and microstrip construction of microwave multiplexers and filters in electromagnetic communications systems and, more particularly, to the construction of a patch filter element incorporating dielectric and/or ferroelectric material for improved tuning capacity of the filter element.
Electromagnetic communications systems, such as those incorporating a communications satellite encircling the earth, employ electronic components such as filters and multiplexers for establishing communication channels. It is desired that such electronic components be of relatively small physical size and weight so as to facilitate their use in a satellite and, accordingly, patch filter elements constructed as microstrip and stripline components are used to advantage in the communications systems. It is also desirable to have a relatively large tuning range for such electronic components so that one construction of the electronic components can be employed in a variety of communications channels by simply altering a tuning thereof. However, presently available electronic components constructed of patch filter elements have a disadvantage in that they are not as readily tunable as would be desired, and may be limited over the range of operating frequency and/or bandwidth that may be desired, particularly in the situation where electronic tuning capability is desired.
The aforementioned disadvantage is overcome and other advantages are provided by a construction of a patch filter element wherein the patch is provided with one or more slots which are filled with a ferroelectric oxide. The patch with the one or more slots constitutes a slotted patch resonator. The slotted patch is constructed with a square or rectangular configuration, allowing operation as a dual mode resonator. The ferroelectric oxide has a dielectric constant significantly higher than the dielectric constant of air and, therefore, results in an increase in capacitive coupling between adjacent sections of the patch separated by the slot. The amount of the ferroelectric oxide placed in a slot is selected to provide a desired amount of capacitive coupling across the slot. For example, the extent of the filling of a slot along its longitudinal dimension can be selected to provide a desired amount of capacitance and coupling. Alternatively, in an embodiment of the invention wherein the foil is provided with depth at the sides of a slot, the amount of the depth of the filling of the slot with the ferroelectric oxide can be selected to provide a desired amount of capacity and coupling. The amount of capacitance between adjacent sections of the patch resonator establishes the resonant frequency of a mode of electromagnetic vibration, and thus, the electrical length of an edge of the patch.
The width of a slot is substantially less than one-half of the free-space wavelength of an electromagnetic signal applied to the patch filter element. Therefore, the presence of the narrow slot does not interfere with the capacity of the patch to resonate at a microwave frequency, but the presence of an elevated dielectric constant of material within the slot alters the resonant wavelength for signals applied to the patch filter element. In a preferred embodiment of the invention, the configuration of a crossed slot is employed centered within a square or rectangular shaped patch, the slots serving to divide the patch into four triangular regions. The slots serve for tuning the filter element and for coupling electromagnetic energy between adjacent ones of the triangular regions. In order to operate the patch filter element with resonance in two orthogonal modes, a discontinuity or asymmetry in the construction of the patch is provided for coupling between orthogonal modes of vibration of electromagnetic waves, a convenient form of such coupling device being the creation of a shoulder by cutting off a corner of the patch.
By virtue of the crossed slot, the four triangular regions are electrically insulated from each other. In a preferred form of construction of the patch filter element, the four triangular regions are constructed of electrically-conductive metallic foil or coating supported on a substrate such as a layer of dielectric insulating material which, in turn, is located on a ground plane. The ground plane may be a layer of electrically-conductive metallic foil. In accordance with a feature of the invention, it is noted that the application of electric fields between adjacent ones of the triangular regions alters the electrical characteristics of the ferroelectric oxide resulting in a shift of the resonant wavelength along an edge of the patch filter element. Thus, the resonant wavelength can be established by choice of length of the side of the patch as well as by a choice of a bias voltage impressed between adjacent triangular regions of the patch. The bias voltage provides for a fringing field across a narrow gap of the slot, this configuration of electric fields providing for accurate control of the dielectric properties of the ferroelectric material in the slot. This form of construction maintains a high Q (quality factor) by a quasi-TEM (transverse electromagnetic) mode of the patch resonator.
Thereby, in the utilization of the slotted patch for construction of a filter, such as a passband filter, one can choose resonant frequencies for sections of the filter electrically by choice of the impressed voltages. Additional sections of the filter can be constructed by employing additional slotted patches, and tuning of the filter can be accomplished electrically. The amount of coupling between adjacent triangular regions of a patch is selected by choice of width of a slot as well as the amount of ferroelectric material present in the slot, wherein a greater slot width reduces the coupling and a smaller slot width increases the coupling. Since the coupling is one of the parameters in development of the overall spectral characteristics of the filter, the availability of choice in coupling in combination with the electronic tuning allows for establishment of a desired spectral characteristic of the filter.